Experimental investigation on mean crushing stress characterization of carbon–epoxy plies under compressive crushing mode

A challenge in numerical simulation of crashworthiness study is to be able to predict the crush damage modes, their evolution during crushing and the energy absorption in any composite structure from elementary material characterisation data. Therefore, it is important to know the behavior of one ply subjected to crushing load, and especially to determine the mean crushing stress that could be used for simulation. For that purpose, quasi-static crushing tests are performed for different configurations of two CFRP materials, UD and fabrics to determine the mean crushing stress of plies alone and inside a laminate. This study shows there is a linear relationship between crushing load and the contact surface of the plies being crushed on a metallic base which enables actually to define a mean crushing stress for a ply. The method to calculate this value is presented in this paper, based on image analysis of specific crushing tests. Experimental results show that for the UD material, the mean crushing stresses in 0° and 90° plies are very close. The value for a balanced fabric is also similar, approximately 270 MPa

Very large magnetoresistance in Fe0.28_{0.28}TaS2_{2} single crystals

Magnetic moments intercalated into layered transition metal dichalcogenides are an excellent system for investigating the rich physics associated with magnetic ordering in a strongly anisotropic, strong spin-orbit coupling environment. We examine electronic transport and magnetization in Fe$_{0.28}$TaS$_{2}$, a highly anisotropic ferromagnet with a Curie temperature $T_{\mathrm{C}} \sim 68.8~$K. We find anomalous Hall data confirming a dominance of spin-orbit coupling in the magnetotransport properties of this material, and a remarkably large field-perpendicular-to-plane MR exceeding 60% at 2 K, much larger than the typical MR for bulk metals, and comparable to state-of-the-art GMR in thin film heterostructures, and smaller only than CMR in Mn perovskites or high mobility semiconductors. Even within the Fe$_x$TaS$_2$ series, for the current $x$ = 0.28 single crystals the MR is nearly $100\times$ higher than that found previously in the commensurate compound Fe$_{0.25}$TaS$_{2}$. After considering alternatives, we argue that the large MR arises from spin disorder scattering in the strong spin-orbit coupling environment, and suggest that this can be a design principle for materials with large MR.Comment: 8 pages, 8 figures, accepted in PR

Pressure-dependent 13C chemical shifts in proteins: Origins and applications

Pressure-dependent (13)C chemical shifts have been measured for aliphatic carbons in barnase and Protein G. Up to 200 MPa (2 kbar), most shift changes are linear, demonstrating pressure-independent compressibilities. CH(3), CH(2) and CH carbon shifts change on average by +0.23, -0.09 and -0.18 ppm, respectively, due to a combination of bond shortening and changes in bond angles, the latter matching one explanation for the gamma-gauche effect. In addition, there is a residue-specific component, arising from both local compression and conformational change. To assess the relative magnitudes of these effects, residue-specific shift changes for protein G were converted into structural restraints and used to calculate the change in structure with pressure, using a genetic algorithm to convert shift changes into dihedral angle restraints. The results demonstrate that residual (13)C alpha shifts are dominated by dihedral angle changes and can be used to calculate structural change, whereas (13)C beta shifts retain significant dependence on local compression, making them less useful as structural restraints

Electrospinning of ultra-thin polymer fibers

The electrospinning technique was used to spin ultra-thin fibers from several polymer/solvent systems. The diameter of the electrospun fibers ranged from 16 nm to 2 μm. The morphology of these fibers was investigated with an atomic force microscope (AFM) and an optical microscope. Polyethylene oxide) (PEO) dissolved in water or chloroform was studied in greater detail. PEO fibers spun from aqueous solution show a “beads on a string” morphology. An AFM study showed that the surface of these fibers is highly ordered. The “beads on a string” morphology can be avoided if PEO is spun from solution in chloroform; the resulting fibers show a lamellar morphology. Polyvinylalcohol (PVA) dissolved in water and cellulose acetate dissolved in acetone were additional polymer/solvent systems which were investigated. Furthermore, the electrospinning process was studied: different experimental lay-outs were tested, electrostatic fields were simulated, and voltage - current characteristics of the electrospinning process were recorded

Crevasse patterns and the strain-rate tensor: a high-resolution comparison

Values of the strain-rate tensor represented at a 20 m length scale are found to explain the pattern and orientation of crevasses in a 0.13 km2 reach of Worthington Glacier, Alaska, U.S.A. The flow field of the reach is constructed from surveyed displacements of 110 markers spaced 20-30 m apart. A velocity gradient method is then used to calculate values of the principal strain-rate axes at the nodes of a 20 m x 20 m orthogonal grid. Crevasses in the study reach are of two types, splaying and transverse, and are everywhere normal to the trajectories of greatest (most tensile) principal strain rate. Splaying crevasses exist where the longitudinal strain rate (Ex) is less than or equal to 0 and transverse crevasses are present under longitudinally extending flow (i.e. Ex greater than 0). The orientation of crevasses changes in the down-glacier direction, but the calculated rotation by the flow field does not account for this change in orientation. Observations suggest that individual crevasses represent local values of the regional flow field and are transient on the time-scale of 1-2 years; they are not persistent features that are translated and rotated by flow. Crevasse patterns are thus found to be a useful tool for mapping the strain-rate tensor in this reach of a temperate valley glacier

Energy Absorption Mechanisms in Layer-to-Layer 3D Woven Composites

3D woven composites provide improved out-of-plane performance over their two-dimensional counterparts. This sort of reinforced through thickness behaviour is desirable in crashworthiness applications where energy absorption can be increased by the composite material's resistance to delamination. The behaviour of these 3D materials in not well understood and fundamental data that can be used to validate and improve material models is not yet sufficiently comprehensive. Here we demonstrate that a modified layer-to-layer type 3D woven architecture can be effectively used in energy absorbing elements to produce repeatable and predictable progressive failure under axial crush conditions. Specific energy absorption (SEA) values in glass and carbon coupons of up to 62J/g and 95J/g respectively are achieved in the quasi-static regime; values up 93J/g to were achieved in the dynamic regime when carbon coupons are tested. Carbon specimens displayed uncharacteristic mixed mode failure with elements of ductile and brittle failure. The addition of a toughening agent showed mixed results in this study, providing quasi-static improvements (+8%) in SEA but significant diminishment in dynamic SEA (-22%). The failure modes present in all cases are explored in depth and the suitability of this material for industry crash applications is investigated

Development of an innovative procedure to assess the crashworthiness of composite materials

L'abstract è presente nell'allegato / the abstract is in the attachmen

Improved crush energy absorption in 3D woven composites by pick density modification

Although 3D woven composites have exceptional out-of-plane properties, there is a lack of understanding for these materials in crash application in automotive and aerospace industries. To encourage the use of 3D wovens in crashworthy automotive structures, knowledge must be gained so that designers can adjust the highly flexible weave parameters to create tailor-made performance materials. Here we show that fabric pick density causes large changes in progressive failure modes and associated energy absorption, particularly in the dynamic regime, where the quasi-static to dynamic energy absorption loss typical of composites is completely removed. Compression and flexure properties, which are known to be linked to crash performance in composites, are also investigated for these 3D woven layer-to-layer interlock carbon-epoxy composite structures. 3D fabric preforms are manufactured in three different pick densities: 4, 10 & 16 wefts/cm. With a constant warp density of 12 warps/cm from carbon fibres. Increasing the pick density improved specific energy absorption (SEA) even in relatively inefficient progressive failure modes like folding, which has not previously observed in composite materials. SEA values up to 104 J/g (quasi-static) and 93 J/g (dynamic) are recorded. This work shows that minor weft direction (transverse) weave changes can lead to sizeable improvements in warp direction (axial) energy absorption without fundamentally redesigning the weave architecture